Generally, a fuel tank of a vehicle with an internal-combustion engine as a driving source is disposed near an exhaust pipe in many cases and, for this reason, the fuel tank is heated by the heat from the exhaust pipe, etc. Moreover, the surface of the fuel in the fuel tank tends to change due to a shake and vibration while a vehicle is moving. From such a situation, an evaporative emission of fuel (henceforth, may be referred to as “fuel vapor”) is likely to be generated in a fuel tank. For this reason, a passage communicated with the external world, the (henceforth, may be referred to as an “air line”), etc. is provided in a fuel tank in order to avoid the rise of the pressure in the fuel tank resulting from fuel vapor. However, it is not preferable that fuel vapor is emitted into the atmosphere (henceforth, may be referred to as an “evaporative emission”) from a viewpoint of earth environment protection and/or safety and health.
Then, in order to prevent emitting fuel vapor into the atmosphere, it is known to interpose a canister which houses adsorption material, such as activated carbon, in an air line to make the adsorption material adsorb fuel vapor. When using such a canister, since there is a limit in the adsorption capability of adsorption material, what is referred to as a “purge processing” that desorbs fuel vapor from adsorption material is needed before adsorption material is saturated with fuel vapor. Generally, for example, as shown in FIG. 1, the purge processing is carried out by desorbing fuel vapor from adsorption material with the negative pressure produced in the intake system 20 during the operation of an internal-combustion engine and/or a suction pump (not shown) provided separately, etc. and introducing purge gas containing this desorbed fuel vapor into the intake system 20 of the internal-combustion engine. In FIG. 1, the outlined white arrow expresses an atmosphere (new air) flow, the arrow of a dotted line expresses the flow of fuel vapor, and the arrow of a solid line expresses the flow of the purge gas (fuel vapor+new air), respectively.
On the other hand, from increasing consciousness to earth environment protection in these days, it is becoming more active to mix alcohol of vegetable origin referred to as “biomass ethanol” and “bioethanol” to conventional gasoline and use as fuel for an internal-combustion engine which is carried, for example, in a vehicle etc. As materials of biomass ethanol, vegetable resources containing a high proportion of carbohydrates or starchy are regarded as suitable and, at present, for example, molasses originating in sugarcane (mainly South America), corn (mainly U.S.), beet (mainly Europe), etc. serve as the main materials of biomass ethanol. Moreover, development of the biomass ethanol which uses plants other than these as materials is also being furthered. Biomass ethanol is positioned as fuel friendly to earth environment based on the view referred to as “carbon-neutral” that even if the fuel which uses these plants as materials is burned and CO2 is generated, it does not necessarily increases the absolute quantity of CO2 of the whole earth, since these plants absorb CO2 in the atmosphere.
As vehicles which can use alcohol blended fuel obtained by mixing bioethanol and conventional gasoline as mentioned above as fuel of an internal-combustion engine, for example, flexible fuel vehicles (FFV), such as an ethanol flexible fuel vehicle, can be mentioned. For example, an ethanol FFV can use alcohol blended fuel which comprises ethanol and gasoline mixed at various ratios as fuel of an internal-combustion engine. In such FFV, a volatilized alcohol component is also contained in the fuel vapor generated in a fuel tank of an internal-combustion engine. Therefore, when using such alcohol blended fuel, the adsorption material of the above-mentioned canister adsorbs fuel vapor containing an alcohol component.
By the way, since alcohol including ethanol has a hydroxy group (OH group) which is a polar group, their polarity is strong, and an alcohol component is more likely to be adsorbed by adsorption material in a canister and less likely to be desorbed, as compared with conventional fuel components, such as gasoline. As a result, for example, as shown in FIG. 2, an alcohol component is likely to accumulate in the adsorption material near the entrance from the tank line 31 of the canister 20 (high concentration domain 36), and the concentration distribution of the alcohol component within the canister 30 becomes uneven. In FIG. 2, the arrow of a black solid line expresses the flow of the fuel vapor which flows from the tank line 31 into the canister 30. Moreover, the domains 37 and 38 express an intermediate concentration domain and a low concentration domain, respectively. In addition, the reference signs 32 and 33 in the figure express an air line and a purge line, respectively. Thereby, the ratio of an alcohol component in the gas introduced into an intake system at the time of a purge processing (henceforth, may be referred to as “purge gas”) and the concentration as the whole fuel vapor contained in the purge gas change with advance of the purge processing.
Moreover, when the ratio of an alcohol component in alcohol blended fuel changes, the ratio of the alcohol component in the fuel vapor to be adsorbed by adsorption material in a canister also changes accordingly. Since an alcohol component is less likely to be desorbed from adsorption material as compared with gasoline etc. as mentioned above, when the ratio of the alcohol component in the fuel vapor to be adsorbed by the adsorption material in a canister changes, the desorption rate (amount of desorption) as the whole fuel vapor at the time of a purge processing of the canister will also change. For this reason, when the ratio of an alcohol component in alcohol blended fuel is changed, by using alcohol blended fuel with a different ratio of an alcohol component, or as the ratio of an alcohol component falls by evaporation in prolonged storage, etc., the quantity of the fuel vapor which desorbs from adsorption material at the time of a purge processing of a canister changes and it becomes difficult to estimate correctly the quantity of the fuel vapor introduced into an intake system.
As mentioned above, an alcohol component in alcohol blended fuel has high polarity, is likely to be adsorbed by adsorption material in a canister and less likely to be desorbed. As a result, an uneven concentration distribution of the alcohol component within the canister will arise, and the concentration as the whole fuel vapor and/or the ratio of the alcohol component which are contained in purge gas will change with advance of the purge processing. Moreover, when the ratio of an alcohol component in alcohol blended fuel is changed, the ratio of the alcohol component in the fuel vapor to be adsorbed by adsorption material in a canister will be also changed. As a result, since the desorption rate (the amount of desorption) as the whole fuel vapor at the time of a purge processing of the canister also changes, it becomes difficult to estimate correctly the quantity of the fuel vapor introduced into an intake system at the time of a purge processing of the canister. From the above, there is a possibility that deviation of an air/fuel ratio may arise and it may become impossible to suppress effectively toxic substance contained in exhaust gas (emission suppression).
On the other hand, in the vehicles with a continuously variable transmission (CVT: Continuously Variable Transmission) mounted thereon, an internal-combustion engine is often used in a high load operating range, and intake negative pressure is small in this range. Furthermore, in a hybrid vehicle (HV), since it comprises a driving source other than an internal-combustion engine (for example, electric motor etc.), the operation period of an internal-combustion engine is short or there is a few opportunities to operate the engine and, as a result, there is a few opportunities itself to carry out a purge processing since there is a few opportunities for an internal-combustion engine to fully be warmed up. Therefore, especially in these vehicles, it is difficult to certainly desorb fuel vapor adsorbed by adsorption material in a canister, and fuel vapor containing an alcohol component is likely to remain adsorbed by adsorption material. As a result, there is a possibility that adsorption material may be saturated with fuel vapor, fuel vapor from a fuel tank bypasses a canister without being once adsorbed in a canister (canister breakthrough) to flow out into the atmosphere (that is, “evaporative emission suppression” may get worse).
As the above, in an internal-combustion engine which uses alcohol blended fuel, an alcohol component contained in fuel vapor has a high polarity and therefore is more likely to be adsorbed by adsorption material in a canister and less likely to be desorbed, as compared with conventional fuel components, such as gasoline. As a result, there is a possibility that above-mentioned “emission suppression” and “evaporative emission suppression” may get worse. However, in the art, although measures against the problem that the alcohol component contained in fuel vapor from alcohol blended fuel degrades the material of the outer shell (for example, microcapsule etc.) of thermal storage medium disposed in a canister to reduce its thermal storage effect (for example, refer to Patent Literature 1 (PTL 1) and Patent Literature 2 (PTL 2)) and measures against the problem that fuel vapor becomes more likely to be generated due to the azeotropy of an alcohol component and a gasoline component and thereby the fuel concentration in purge gas becomes too high at the start of a purge processing of a canister (for example, refer to Patent Literature 3 (PTL 3)) have been proposed, it is a reality that any effective measure against the aggravation of the emission suppression and/or evaporative-emission suppression resulting from deviation of an air/fuel ratio produced due to the high polarity of the alcohol component as mentioned above, has not yet been proposed.